<h3>Hello WebGL</h3>
<ul>
    <li>清空背景</li>
</ul>
<div class="color-box"></div>
<span>选择背景颜色:</span>
<button>黑色</button>
<button>白色</button>
<button>红色</button>
<button></button>
<div class="canvas-box">
    <canvas #canvas width="300" height="300"></canvas>
</div>